The present invention relates to optical switches employing controllable phase-mismatched coupling between optical waveguides arranged on a substrate, and more particularly to an optical switch, in which a plurality of optical switch elements are integrated on a single substrate, for electrically controlling and switching as desired the path of light between a plurality of input and output optical waveguides.
Such an optical switch is intended for use in a multichannel optical switch for switching optical signals in time series and multiplexing them for transmission over a single optical fiber transmission path or, conversely, distributing by time division a plurality of optical signals sent over a single fiber to a plurality of terminals, or for use in an optical switching network in which a matrix is so composed that a plurality of transmission paths can be connected to one another as desired.
As optical communication systems are increasingly used in practical application, there are continued efforts to develop further sophisticated systems having greater capacities and/or more diverse functions. There are in demand new systems which can carry out such functions as exchange between optical transmission networks and high-speed connection and switching between the terminals of optical data buses. The need for optical switches to make possible these functions is accordingly growing.
Currently available optical switches for practical use include those using mechanical shift by electromagnets or the like. There, however, are not fully satisfactory in respect to speed, multi-point switching ability and reliability.
Therefore work is under way to develop guided wave type optical switches which are satisfactory in all these respects and, moreover, are highly efficient, compact and adaptive to single mode fiber systems. A guided wave type optical switch, in which are used optical wave guides arranged on a substrate, has the advantage that a plurality of optical switch elements can be integrated on a single substrate, and accordingly a matrix type optical switch of guided wave design can be constructed with comparative ease.
The directional coupler type, total internal reflection type, Bragg diffraction type and Y-branching type structures have so far been proposed for guided wave optical switches. The first two permit relatively ready suppression of optical losses and crosstalks, both vital parameters for optical switches, and are simple enough in structure to allow the realization of multi-channel or matrix design.
A total internal reflection type switch has two optical waveguides crossing each other at an angle of several degrees. Controlling electrodes are provided at their intersection to control light reflection at the intersection. For an example of a matrix type optical switch of the total internal reflection design, see C. L. Chang, F. R. El-Aklari and C. S. Tsai, "Fabrication and Testing of Optical Channel Waveguide Total Internal Reflection (TIR) Switching Networks Proceedings of the Society of Photo-Optical Instrumentation Engineers, Vol. 239, Guided-Wave Optical and Surface Acoustic Wave Devices, Systems and Applications (1980), pp. 147-151. As a TIR switch requires an appreciable angle of intersection to achieve sufficiently low crosstalk, a substantial applied voltage is necessary. Since it usually is difficult to construct a high-voltage and yet high-speed driving circuit, a TIR switch is unsuitable for high-speed switching.
Meanwhile, in a directional coupler type optical switch, two optical waveguides of several microns to several tens of microns in width are arranged close to each other at a distance of several microns to constitute an optical directional coupler, and the coupling of said two optical waveguides is controlled by applying voltages on control electrodes provided in the vicinity of the optical waveguides. This type optical switch operates at lower voltages and can achieve a low level of crosstalk with greater ease than optical switches of other types. (See, for example, R. V. Schmidt and L. L. Buhl, "Experimental 4.times.4 Optical Switching Network", Electronics Letters (1976), Vol. 12, No. 22, p. 575.)
However, this type switch requires two different voltages, V.sub.1 (.noteq.0) and V.sub.2 (.noteq.0), to drive each optical switch element and, moreover, the voltages somewhat vary from element to element, so that a complex driving circuit is required. For high-speed driving of an optical switch, low voltages are desirable. Further, for achieving a low crosstalk of -20 dB or below which is required by the system, the optical waveguides and electrodes have to be perfect, the symmetry between the two optical waveguides has to be highly precise. However, meeting these requirement is a difficult task.
Moreover, in a conventional matrix type optical switch in which many pairs of input and output ports are connected in a certain combination, if any two pairs out of them are to be interchanged, the connections of a great number of optical switch elements will have to be altered at the same time. In other words, since the light path within the matrix optical switch has to be substantially changed when only two pairs of input and output ports are to be interchanged, there is involved the disadvantage that the connections between other input and output ports are momentarily disturbed or the output light levels of other output ports instantaneously drop at the time of switching.